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Internal variability in twenty-first-century summer Arctic sea ice loss and its relationship to the large-scale atmospheric circulation is investigated in a 39-member Community Climate System Model, version 3 (CCSM3) ensemble for the period 2000-61. Each member is subject to an identical greenhouse gas emissions scenario and differs only in the atmospheric model component's initial condition. September Arctic sea ice extent trends during 2020-59 range from −2.0 × 106 to −5.7 × 106 km2 across the 39 ensemble members, indicating a substantial role for internal variability in future Arctic sea ice loss projections. A similar nearly threefold range (from −7.0 × 103 to −19 × 103 km3) is found for summer sea ice volume trends. Higher rates of summer Arctic sea ice loss in CCSM3 are associated with enhanced transpolar drift and Fram Strait ice export driven by surface wind and sea level pressure patterns. Over the Arctic, the covarying atmospheric circulation patterns resemble the so-called Arctic dipole, with maximum amplitude between April and July. Outside the Arctic, an atmospheric Rossby wave train over the Pacific sector is associated with internal ice loss variability. Interannual covariability patterns between sea ice and atmospheric circulation are similar to those based on trends, suggesting that similar processes govern internal variability over a broad range of time scales. Interannual patterns of CCSM3 ice-atmosphere covariability compare well with those in nature and in the newer CCSM4 version of the model, lending confidence to the results. Atmospheric teleconnection patterns in CCSM3 suggest that the tropical Pacific modulates Arctic sea ice variability via the aforementioned Rossby wave train. Large ensembles with other coupled models are needed to corroborate these CCSM3-based findings. [ABSTRACT FROM AUTHOR]

Two important dynamical processes influence the extratropical zonal wind field: angular momentum transport by the thermally direct Hadley circulation (thermal-driving T) and momentum flux convergence by atmospheric waves (eddies) that develop in regions of enhanced baroclinicity (eddy-driving E). The relationship between extratropical zonal wind variability and these driving processes is investigated using 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) data. Indices representing the processes (iT and iE) are defined based on vertically integrated diabatic heating and meridional convergence of the meridional flux of zonal momentum by eddies, respectively. Zonal wind signatures associated with these indices are identified via composite analysis. In the Atlantic sector, zonal wind variability is mainly associated with momentum flux convergence by baroclinic eddies, supporting the established view that the Atlantic jet is primarily eddy driven. In the Pacific sector, zonal wind variability is associated with both driving processes, evidence that the Pacific jet is both thermally driven and eddy driven. The thermally driven Pacific signature reflects changes in jet strength (intensity and longitudinal extent) with some resemblance to the zonal wind anomalies of the Pacific-North America (PNA) pattern. The eddy-driven signature reflects a latitudinal shift of the jet exit region in both sectors that resembles the zonal wind anomalies of the North Atlantic Oscillation (NAO) or West Pacific (WP) patterns. [ABSTRACT FROM AUTHOR]

Patterns of correlation between tree rings and local temperature or precipitation are investigated using 762 International Tree-Ring Data Bank standardized ring width site chronology time series, and a gridded dataset of temperature and precipitation. Coherent regional- and, in some cases, hemispheric-scale patterns of correlation are found in the extratropical Northern Hemisphere for both the summer prior to and the summer concurrent with ring width formation across different species and over large distances. Among those chronologies that are primarily linked to temperature, thicker ring widths are generally associated with anomalously cool prior summer temperature and anomalously warm concurrent summer temperature. Reconstructions of local summer temperature using prior, concurrent, and/or subsequent year ring widths as predictors demonstrate that useful climate-growth information generally exists in ring widths that are both concurrent with and subsequent to the summer temperature anomaly. Consistent prior summer temperature-ring width relationships have received relatively little previous attention. Among those chronologies that are primarily linked to precipitation, thicker ring widths are generally associated with high summer precipitation in both the year prior to and the year concurrent with ring formation. The magnitude and spatial consistency of temperature correlations are greater than those for precipitation, at least on the hemispheric scale. These results support and serve to generalize the conclusions of prior regionally restricted and/or species-specific studies relating ring width to energy and/or water limitations. Regional- and hemispheric-scale patterns of ring width-temperature or ring width-precipitation correlations show up more clearly in species-specific and frequency-dependent analyses. Different species respond differently to temperature and precipitation anomalies. Consistent with the hemispheric patterns described above, most standardized ring width time series more faithfully record the high frequency component of the temperature signal than the low frequency component. The potential for enhanced coherence in regionally restricted, species-specific, and frequency-dependent analyses is independently verified by examining the correlation between ring width time series over geographical distance. This broader characterization of relationships between tree-ring widths and local climate provides an objective basis for selecting tree ring or other similarly high-resolution proxy data for regional-, hemispheric-, or global-scale paleoclimate reconstructions. [ABSTRACT FROM AUTHOR]

Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthly-averaged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as “pulsing” and “latitudinal shifting” of the climatological-mean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent. Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)–Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of storm-track and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific–North America (PNA) patterns. Wave–mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet. [ABSTRACT FROM AUTHOR]

An analysis of detailed relationships between the North Atlantic Oscillation–Arctic Oscillation (NAO–AO) and local temperature response throughout the northeastern United States and neighboring areas of Canada is presented. In particular, the study focuses on how contrasts in the mean and daily variance, based on AO phase, are associated with contrasts in the frequency and intensity of extreme temperature events in both winter and spring. In this region, notable contrasts in mean temperatures in winter and daily variance in spring, which influence the pattern of extremes, are associated with phases of the NAO–AO. Warmer temperatures in New England and cooler temperatures in Quebec, Canada, result during winter with increases in the NAO–AO index. The mean temperature response is weaker in spring, but the response of daily variance of temperature is stronger; variance increases with the NAO–AO index. Relationships between these effects help explain significant increases in maximum temperature extremes during winter in New England and in minimum temperature extremes during spring in Quebec for high NAO–AO index years. Diurnal temperature range tends to be larger in AO-positive winters and springs throughout the region. This study helps put other work on the trends in regional extreme events into the context of large-scale climate variability. [ABSTRACT FROM AUTHOR]

High‐resolution regional climate model (RCM) simulations of global warming consistently predict larger percentage increases in precipitation in the lee of midlatitude mountain ranges than on their windward slopes, indicating a weakening of the orographic rain shadow. This redistribution of precipitation could have profound consequences for water resources and ecosystems, but its underlying mechanisms are unknown. Here we show that rain‐shadow weakening is just one manifestation of a more general decrease in the influence of orography on precipitation under global warming. We introduce a simple model of precipitation change based on this principle, and find that it agrees well with an ensemble of high‐resolution simulations performed over the western United States. We argue that diminished orographic influence can be explained by the unique vertical structure of orographically forced ascent, which tends to maximize in the lower atmosphere where condensation is thermodynamically less sensitive to warming. Plain Language Summary: Mountains supply about half of the fresh water used by humans globally and often regulate the seasonal cycle of stream flows by accumulating snowpack during the wet season and releasing it over the dry season. Regional simulations tend to predict large future changes in the spatial pattern of precipitation in mountainous regions, but the reasons for these changes are not well understood. Here we show that the predicted patterns of precipitation change in the western US can be explained by a simple mechanism that causes the influence of mountains to weaken with increasing temperatures. We introduce a simple model of precipitation change based on this effect, and show that its predictions closely match those of far more sophisticated regional simulations of future warming over the western US. Key Points: Orographic influence on precipitation is reduced in simulations of global warmingFractional precipitation change scaled by warming is reproducible with a simple model [ABSTRACT FROM AUTHOR]

This study investigates the sensitivity of the North Atlantic storm track to future changes in local and global sea surface temperature (SST) and highlights the role of SST changes remote to the North Atlantic. Results are based on three related coupled climate models: the Community Climate System Model, version 4 (CCSM4), the Community Earth System Model, version 1 (Community Atmosphere Model, version 5) [CESM1(CAM5)], and the Norwegian Earth System Model, version 1 (intermediate resolution) (NorESM1-M). Analysis reveals noticeable intermodel differences in projected storm-track changes from the coupled simulations [i.e., the difference in 200-hPa eddy activity between the representative concentration pathway 8.5 (RCP8.5) and historical scenarios]. In the CCSM4 coupled simulations, the North Atlantic storm track undergoes a poleward shift and eastward extension. In CESM1(CAM5), the storm-track change is dominated by an intensification and eastward extension. In NorESM1-M, the storm-track change is characterized by a weaker intensification and slight eastward extension. Atmospheric experiments driven only by projected local (North Atlantic) SST changes from the coupled models fail to reproduce the magnitude and structure of the projected changes in eddy activity aloft and zonal wind from the coupled simulations. Atmospheric experiments driven by global SST and sea ice changes do, however, reproduce the eastward extension. Additional experiments suggest that increasing greenhouse gas (GHG) concentrations do not directly influence storm-track changes in the coupled simulations, although they do through GHG-induced changes in SST. The eastward extension of the North Atlantic storm track is hypothesized to be linked to western Pacific SST changes that influence tropically forced Rossby wave trains, but further studies are needed to isolate this mechanism from other dynamical adjustments to global warming. [ABSTRACT FROM AUTHOR]

The decline in Barents Sea ice has been implicated in forcing the 'warm-Arctic cold-Siberian' (WACS) anomaly pattern via enhanced turbulent heat flux (THF). This study investigates interannual variability in winter [December-February (DJF)] Barents Sea THF and its relationship to Barents Sea ice and the large-scale atmospheric flow. ERA-Interim and observational data from 1979/80 to 2011/12 are used. The leading pattern (EOF1: 33%) of winter Barents Sea THF variability is relatively weakly correlated ( r = 0.30) with Barents Sea ice and appears to be driven primarily by atmospheric variability. The sea ice-related THF variability manifests itself as EOF2 (20%, r = 0.60). THF EOF2 is robust over the entire winter season, but its link to the WACS pattern is not. However, the WACS pattern emerges consistently as the second EOF (20%) of Eurasian surface air temperature (SAT) variability in all winter months. When Eurasia is cold, there are indeed weak reductions in Barents Sea ice, but the associated THF anomalies are on average negative, which is inconsistent with the proposed direct atmospheric response to sea ice variability. Lead-lag correlation analyses on shorter time scales support this conclusion and indicate that atmospheric variability plays an important role in driving observed variability in Barents Sea THF and ice cover, as well as the WACS pattern. [ABSTRACT FROM AUTHOR]

A new approach is put forward for defining extratropical teleconnection patterns. The zonal wind field at 250 hPa is analyzed separately in the North Atlantic and North Pacific Ocean sectors during the winter season (December–March). Teleconnectivity of this field is found to be particularly strong. EOF analysis of the zonal wind field yields patterns that (i) are robust with respect to the range of frequencies included in the data, (ii) relate clearly to the position of the climatological-mean jets, and (iii) are broadly consistent with their traditionally defined counterparts in terms of climatic impacts. The patterns are characterized by a north–south shifting or an extension/retraction of the eddy-driven jet in its exit region and similar changes at the entrance region of the subtropical jet. The patterns also reflect the degree of separation between the subtropical and eddy-driven jets. Atlantic EOFs 1 and 2 are counterparts of the North Atlantic Oscillation (NAO) and eastern Atlantic pattern, respectively, while Pacific EOF 1 is the counterpart of the Pacific–North America (PNA) pattern. Pacific EOF 2, a pattern that has not been previously noted, has a pronounced impact on the jet configuration and precipitation over the western coast of North America. This pattern may be of particular interest for precipitation forecasting applications. Atlantic EOF 1 exhibits a long decorrelation time and strong negative skewness. The relation between these jet variability patterns and the storm-track variability is examined, including the dynamical interaction between baroclinic waves and the jets. In each sector, the eddy forcing is found to maintain the respective jet anomalies. [ABSTRACT FROM AUTHOR]

Jet streams shape midlatitude weather and climate. The North Atlantic jet is mainly eddy‐driven, with frequent north–south excursions on synoptic time scales arising from eddy forcings and feedbacks. There are, however, special periods during which the underlying dynamics appear to change—for example, winter 2009/2010, when the jet was persistently southward‐shifted, extremely zonal, and more thermally driven. This study shows evidence that the southern jet configuration exhibits altered dynamical behavior involving a shift in the balance of thermal and eddy‐driving processes, independent of timescale. Specifically, southern jets exhibit weaker eddy feedbacks and are associated with enhanced heating in the tropical Pacific. During winter 2009/2010, a remarkably frequent (66 days out of the 90‐day winter season) and persistent southern jet shaped the unusual seasonal signature. These results bridge the synoptic and climate perspectives of jet variability, with potential to help understand and reduce biases in regional climate variability as simulated by models. [ABSTRACT FROM AUTHOR]

Understanding the unfolding challenges of climate change relies on climate models, many of which have large summer warm and dry biases over Northern Hemisphere continental midlatitudes. This work, with the example of the model used in the updated version of the weather@home distributed climate model framework, shows the potential for improving climate model simulations through a multiphased parameter refinement approach, particularly over the northwestern United States (NWUS). Each phase consists of (1) creating a perturbed parameter ensemble with the coupled global–regional atmospheric model, (2) building statistical emulators that estimate climate metrics as functions of parameter values, (3) and using the emulators to further refine the parameter space. The refinement process includes sensitivity analyses to identify the most influential parameters for various model output metrics; results are then used to cull parameters with little influence. Three phases of this iterative process are carried out before the results are considered to be satisfactory; that is, a handful of parameter sets are identified that meet acceptable bias reduction criteria. Results not only indicate that 74 % of the NWUS regional warm biases can be reduced by refining global atmospheric parameters that control convection and hydrometeor transport, as well as land surface parameters that affect plant photosynthesis, transpiration, and evaporation, but also suggest that this iterative approach to perturbed parameters has an important role to play in the evolution of physical parameterizations. [ABSTRACT FROM AUTHOR]

Jet streams exhibit large variability over a range of time scales, influencing both global climate and regional weather. They steer cyclones in the storm tracks, altering the exchange of heat with the underlying ocean as well as precipitation and evaporation patterns. The North Atlantic jet is mainly eddy-driven, with characteristic north-south fluctuations on synoptic time scales as captured in part by the North Atlantic Oscillation (NAO) index.However, during special periods, the dynamics of the North Atlantic jet can change - for example, during the 2009/2010 winter, it took on the attributes of a merged jet, which is both eddy-driven and thermally driven. The merged North Atlantic jet is extremely zonal, shifted equatorward by about 8° latitude relative to its climatological mean position, and is linked to cold and dry conditions over Northern Europe.In this study, we bridge the weather and climate perspectives and show that such altered dynamical behaviour of the North Atlantic jet occurs not only on longer (monthly or seasonal) time scales, but also on synoptic time scales. We identify weaker eddy forcing and slightly increased persistence in the North Atlantic sector during merged jet periods, associated with changes in the storm track, cold air outbreaks, and atmosphere-ocean heat exchange. In addition, we find indications of enhanced heating over warmer sea surface temperatures in the tropical Pacific that may act as remote source of thermal driving.These results help provide context for interpreting both observed North Atlantic variability from synoptic to seasonal time scales and simulated jet changes under global warming. Our findings also have potential implications for understanding biases in climate models, many of which simulate an overly zonal and extended North Atlantic jet. [ABSTRACT FROM AUTHOR]

Studies of climate effects on ecology often account for non-stationarity in individual physical and biological variables, but rarely allow for non-stationary relationships among variables. Here, we show that non-stationary relationships among physical and biological variables are central to understanding climate effects on salmon (Onchorynchus spp.) in the Gulf of Alaska during 1965- 2012. The relative importance of two leading patterns in North Pacific climate, the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), changed around 1988/1989 as reflected by changing correlations with leading axes of sea surface temperature variability. Simultaneously, relationships between the PDO and Gulf of Alaska environmental variables weakened, and long-standing temperature-salmon and PDO-salmon covariance declined to zero. We propose a mechanistic explanation for changing climate-salmon relationships in terms of non-stationary atmosphere-ocean interactions coinciding with changing PDO-NPGO relative importance. We also show that regression models assuming stationary climate-salmon relationships are inappropriate over the multidecadal time scale we consider. Relaxing assumptions of stationary relationships markedly improved modelling of climate effects on salmon catches and productivity. Attempts to understand the implications of changing climate patterns in other ecosystems might also be aided by the application of models that allow associations among environmental and biological variables to change over time. [ABSTRACT FROM AUTHOR]

Understanding the unfolding challenges of climate change relies on climate models, many of which have large summer warm and dry biases over Northern Hemisphere continental mid-latitudes. This work, using the example of the model used in the updated version of the weather@home distributed climate model framework, shows the potential for improving climate model simulations through a multi-phased parameter refinement approach, particularly over northwestern United States(NWUS). Each phase consists of 1) creating a perturbed physics ensemble with the coupled global -- regional atmospheric model, 2) building statistical emulators that estimate climate metrics as functions of parameter values, 3) and using the emulators to further refine the parameter space. The refinement process includes sensitivity analyses to identify the most influential parameters for various model output metrics; results are then used to cull parameters with little influence. Three phases of this iterative process are carried out before the results are considered to be satisfactory; that is, a handful of parameter sets are identified that meet acceptable bias reduction criteria. Results not only indicate that 74% of the NWUS regional warm biases can be reduced by refining global atmospheric parameters that control convection and hydrometeor transport, and land surface parameters that affect plant photosynthesis, transpiration and evaporation, but also suggest that this iterative approach to perturbed physics has an important role to play in the evolution of physical parameterizations. [ABSTRACT FROM AUTHOR]

This study investigates the global response of the midlatitude atmospheric circulation to 1.5 and 2.0 °C of warming using the HAPPI (Half a degree Additional warming, Prognosis and Projected Impacts) ensemble, with a focus on the winter season. Characterising and understanding this response is critical for accurately assessing the near-term regional impacts of climate change and the benefits of limiting warming to 1.5 °C above pre-industrial levels, as advocated by the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC). The HAPPI experimental design allows an assessment of uncertainty in the circulation response due to model dependence and internal variability. Internal variability is found to dominate the multi-model mean response of the jet streams, storm tracks, and stationary waves across most of the midlatitudes; larger signals in these features are mostly consistent with those seen in more strongly forced warming scenarios. Signals that emerge in the 1.5 °C experiment are a weakening of storm activity over North America, an inland shift of the North American stationary ridge, an equatorward shift of the North Pacific jet exit, and an equatorward intensification of the South Pacific jet. Signals that emerge under an additional 0.5 °C of warming include a poleward shift of the North Atlantic jet exit, an eastward extension of the North Atlantic storm track, and an intensification on the flanks of the Southern Hemisphere storm track. Case studies explore the implications of these circulation responses for precipitation impacts in the Mediterranean, in western Europe, and on the North American west coast, paying particular attention to possible outcomes at the tails of the response distributions. For example, the projected weakening of the Mediterranean storm track emerges in the 2 °C warmer world, with exceptionally dry decades becoming 5 times more likely. [ABSTRACT FROM AUTHOR]

This study investigates the global response of the midlatitude atmospheric circulation to 1.5°C and 2.0°C of warming using the HAPPI Half a degree Additional warming, Projections, Prognosis and Impacts ensemble, with a focus on the winter season. Characterizing and understanding this response is critical for accurately assessing the near-term regional impacts of climate change and the benefits of limiting warming to the 1.5°C above pre-industrial levels, as advocated by the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC). The HAPPI experimental design allows an assessment of uncertainty in the circulation response due to model dependence and internal variability. Internal variability is found to dominate the multi-model mean response of the jet streams, storm tracks and stationary waves across most of the midlatitudes; larger signals in these features are mostly consistent with those seen in more strongly forced warming scenarios. Signals that emerge in the 1.5°C experiment are a weakening of storm activity over North America, an inland shift of the North American stationary ridge, an equatorward shift of the North Pacific jet exit, and an equatorward intensification of the South Pacific jet. Signals that emerge under an additional 0.5°C of warming include a poleward shift of the North Atlantic jet exit, an eastward extension of the North Atlantic storm track, and an intensification on the flanks of the Southern Hemisphere storm track. Case studies explore the implications of these circulation responses for precipitation impacts in the Mediterranean, western Europe and the North American west coast, paying particular attention to possible outcomes at the tails of the response distributions. For example, the projected weakening of the Mediterranean storm track emerges in the 2.0°C world, though the ensemble spread still allows for both wetting and drying responses. [ABSTRACT FROM AUTHOR]

Common approaches for summarizing multivariate environmental or community data assume that relationships among variables are stationary over time, and this assumption is often not tested. Here we test the hypothesis that relationships among environmental and community time series are nonstationary in the Gulf of Alaska ecosystem (North Pacific Ocean) over multidecadal time scales. Dynamic factor analysis (DFA) is applied to environmental and community data from before and after 1988/1989, corresponding to the timing of an abrupt decline in temporal variance of the Aleutian Low atmospheric pattern, a leading driver of Gulf of Alaska climate. Results show that covariance among local atmosphere and ocean environmental variables weakened simultaneous to the decline in Aleutian Low variance. At the same time, community‐wide responses of 14 fish and crustacean populations to physical forcing weakened, as indicated by nonstationary environment–biology regression coefficients. In line with theoretical predictions, this loss of a shared response to environmental variability was accompanied by weakening community covariance. Individual populations also showed nonstationary relationships with shared trends of community variability. We conclude that assumptions of fixed environmental and community relationships are likely to produce mistaken inference in this ecosystem. Similar concerns may apply in other ecosystems subject to changing climate patterns. [ABSTRACT FROM AUTHOR]